****由于我已经创建了一个名为createNode(int(的函数,它将返回结构节点*类型的内存块,但我没有提到return(temp(仍然代码正常工作,操作如插入,删除工作正常,那里是否有堆或堆栈的概念。?****
struct node* createNode(int data){
struct node *temp;
temp = (struct node*)malloc(sizeof(struct node));
temp->data = data;
temp->next = NULL;
// return temp
}
void insertNode(int position){
struct node *temp;
....
temp = createNode(data);
....
}
这是一个未定义的行为。但我仍然会尝试解释为什么你在这里很幸运。
我在提供的代码中添加了更多代码,现在看起来像
/* test.c */
#include<stdio.h>
#include<stdlib.h>
struct node {
int data;
struct node* next;
};
struct node* createNode(int data){
struct node *temp;
temp = (struct node*)malloc(sizeof(struct node));
temp->data = data;
temp->next = NULL;
// return temp
}
int main(){
struct node *temp = createNode(12);
printf("%d %x", temp->data);
}
编译:
$ gcc -g test.c
使用 gdb 运行它,以便您可以看到反汇编
$ gdb -q ./a.out
Reading symbols from /root/a.out...done.
反汇编函数 createNode 以查看malloc
返回值的位置(因为这是我们将返回回main
的值(。请注意,通常保存函数返回值的 rax 包含 malloc 的返回值(这是您幸运的地方(
(gdb) disass createNode
Dump of assembler code for function createNode:
0x0000000000400580 <+0>: push %rbp
0x0000000000400581 <+1>: mov %rsp,%rbp
0x0000000000400584 <+4>: sub $0x20,%rsp
0x0000000000400588 <+8>: mov %edi,-0x14(%rbp)
0x000000000040058b <+11>: mov $0x10,%edi
0x0000000000400590 <+16>: callq 0x400480 <malloc@plt>
0x0000000000400595 <+21>: mov %rax,-0x8(%rbp) <== rax register contains the return value of malloc, value is pushed to stack
0x0000000000400599 <+25>: mov -0x8(%rbp),%rax <== rax value retrieved from stack. now rax contains the return value of malloc
0x000000000040059d <+29>: mov -0x14(%rbp),%edx
0x00000000004005a0 <+32>: mov %edx,(%rax) <== node->next assignment is done here
0x00000000004005a2 <+34>: mov -0x8(%rbp),%rax <== again rax is populated by return value of malloc
0x00000000004005a6 <+38>: movq $0x0,0x8(%rax) <== node->next is assigned to NULL here.
0x00000000004005ae <+46>: leaveq
0x00000000004005af <+47>: retq
End of assembler dump.
反汇编函数main
以查看如何调用createNode
以及我们从哪里获取返回值。请注意,rax 值被读入主帧中的 temp 变量中。
(gdb) disass main
Dump of assembler code for function main:
0x00000000004005b0 <+0>: push %rbp
0x00000000004005b1 <+1>: mov %rsp,%rbp
0x00000000004005b4 <+4>: sub $0x10,%rsp
0x00000000004005b8 <+8>: mov $0xc,%edi
0x00000000004005bd <+13>: callq 0x400580 <createNode> <== createNode called
0x00000000004005c2 <+18>: mov %rax,-0x8(%rbp) <== rax contains the malloc's return value, so we got the correct value luckily
0x00000000004005c6 <+22>: mov -0x8(%rbp),%rax
0x00000000004005ca <+26>: mov (%rax),%eax
0x00000000004005cc <+28>: mov %eax,%esi
0x00000000004005ce <+30>: mov $0x400670,%edi
0x00000000004005d3 <+35>: mov $0x0,%eax
0x00000000004005d8 <+40>: callq 0x400450 <printf@plt>
0x00000000004005dd <+45>: leaveq
0x00000000004005de <+46>: retq
End of assembler dump.
(gdb) q
我希望这可以解释为什么我们在 temp 中看到正确的值,即使 createNode 中没有返回语句。